My motivation is still the fun and that I learn on the way and from others.

And before I made money from open source, I always did it with the (very tiny) hope of making this a full-time job. Basically I created a dream job for myself. (Of course this wasn't easy and isn't for everyone as this took a long time, talented other people, luck, hard effort, more good than bad decisions, ...)

> I could not just approach my employer and ask if I could help code this open source project.

Why not? (Your chances are probably higher, when the project is related but still you should try.)